The present invention is directed to bicycle transmissions and, more particularly, to internal hub transmissions that use planetary gear mechanisms.
A typical internal hub transmission for a bicycle includes a hub axle for retaining the transmission to a bicycle frame, a driver rotatably supported relative to the hub axle, and an output member rotatably supported relative to the hub axle. The driver usually is mounted with a sprocket for receiving a driving force from a chain connected to the pedal cranks, and the output member usually is a hub shell that is connected to the wheel spokes which, in turn, are connected to the wheel rim. A planet gear mechanism usually is disposed between the driver and the hub shell for communicating the rotational force from the driver to the hub shell through a plurality of transmission paths which change the drive ratio of the transmission. In a three speed hub, for example, the planet gear mechanism may include a first sun gear and a second sun gear rotatably supported relative to the hub axle, an intermediate gear unit rotatably supported relative to the hub axle, wherein the intermediate gear unit meshes with the first and second sun gears, and a ring gear meshing with the intermediate gear unit.
Changing the gear ratio in a planet gear mechanism usually involves selectively stopping the rotation of the first and second sun gears relative to the hub axle. This usually involves coupling the first sun gear to the hub axle using a coupling member interposed between the first sun gear and the hub axle, and then coupling the second sun gear to the hub axle using another coupling member interposed between the second sun gear and the hub axle. Sometimes a single coupling member is used to perform both functions. Such a coupling member usually takes the form of an elongated member that slides along the hub axle, wherein the coupling member initially engages a first stopper on the first sun gear and a first stopper on the hub axle and then slides along the hub axle to that the coupling member engages a second stopper on the second sun gear and a second stopper on the hub axle. If the width of the elongated member is less than the distance between the first stopper on the first sun gear and the second stopper on the second sun gear, there may be a time while the elongated member is being moved from the first sun gear to the second sun gear that the coupling member is disengaged from both the first sun gear and the second sun gear, thus causing a break in the power transmission path between the driver and the hub shell and causing freewheeling of the hub shell.